Optical networks for long haul and metro area communication typically employ wavelength division multiplexing (WDM) or dense wavelength division multiplexing (DWDM) to increase transmission capacity. In WDM and DWDM networks, a number of optical channels are carried in each optical fiber at disparate wavelengths. Network capacity is based on the number of the wavelengths, or channels, in each fiber and the data rates of the channels. Individual channels can be added or dropped at network sites using optical multiplexing and demultiplexing equipment.
Optical amplifiers (OA) such as Erbium-doped fiber amplifiers (EDFA) are commonly used in optical long haul and metro networks to offset loss of light in optical fiber links and transmission and multiplexing/demultiplexing equipment at network sites. In EDFA designs, optical laser diodes, which typically emit at 980 nm or 1480 nm, are used as sources of pump radiation to provide optical energy to amplify weak optical input signals, typically in the 1500 nm to 1600 nm wavelength range.
The power of a WDM signal at the input of an optical amplifier employed in a WDM communication system can vary for a number of reasons. For example, power variations can be caused by an intentional increase or decrease in the number of channels for the purpose of routing traffic, by the unintentional loss of channels due to a fiber cut or human error, changes in span losses, and component loss changes due to aging or temperature fluctuations. The number of optical channels may be varying rapidly depending upon the number of added or dropped channels at a particular network site, resulting in a rapid change in the input optical power into the amplifier. Since the amplifier gain typically varies with the total input signal power, a change in the input optical power typically results in a change in the amplifier gain, unless the pump power is adjusted accordingly. Since the amplifier gain has to exactly offset loss in a transmission system, it is important to maintain a constant amplifier gain as the input optical power into the OA changes. This type of control is commonly referred to as an automatic gain control (AGC) or transient control. It is well known that AGC can be achieved by adjusting the pump power supplied to the optical amplifier. In general, the change in the pump power required to maintain constant gain depends not only on the input signal power level but also on the spectral content of the input signal.
Known techniques for implementing the transient control by controlling the pump power include feed-forward (FF) and feedback (FB) arrangements. In a feed-forward arrangement the pump power may be adjusted based upon changes to the input optical signal into the OA. It offers the advantage of a faster response time, but can also be inaccurate since it does not take into account variations in the amplifier due to aging of various components thereof and with temperature, the dependence of the optimum pump power on the spectral content of the input signal. In a feedback arrangement, parameters used to determine the appropriate pump power include at least one output parameter of the OA, such as the optical power of light at the amplifier output. Typically, the input and output optical signals of the OA may be detected and used to determine the actual gain G of the amplifier. This measured gain may then be used to adjust the pump power until the desired gain is achieved.
FIG. 1 illustrates an exemplary OA 1 having a typical gain control arrangement, wherein circuitry for controlling the pump power includes feed-forward and feedback portions. A WDM optical signal enters the OA 1 through an input optical port 2, is amplified in a gain section 8 of the amplifier, which typically includes a span of an erbium-doped fiber pumped by pump radiation from a pump laser diode (LD) module 10, and leaves the OA 1 from an output port 7, with its power increased in accordance with a current value of the amplifier gain G. The pump power provided by the pump module 10 is controlled by a pump control circuit 5. The FF signal is obtained by tapping off a small portion of the input optical signal using an optical tap 3, and converting it into an electrical signal with an input photodetector (PD) 4. Similarly, the FB signal is obtained by tapping off a small portion of the output optical signal using an output optical tap 9, and converting it into an electrical signal with an output photodetector 6. The pump control circuitry 5 generates a pump control signal based on the FF and FB signals obtained from the PDs 4 and 6, and applies it to the pump module 10, such as to vary the pump current into the pump LD to maintain a constant gain in the OA gain section 8.
One known approach to obtaining the pump control signal based on the input and output power of the OA, as read by the PDs 4 and 6, is to utilize a digital signal processor (DSP) programmed with a suitable control algorithm to control the pump current of the pump LD 10. Such an implementation of the OA control circuitry is illustrated in FIG. 2, and requires using two fast analog to digital converters (ADCs) 11 to convert the analog FF and FB signals from the PDs 4, 6 into the digital domain prior to providing it to the DSP 5, and a fast digital to analog converter (DAC) at the input of the pump control module 10. The DSP 5 may run a firmware-implemented PID (Proportional-Integral-Derivative) control loop, which typically has to be closed faster than 1 microsecond (μsec), and may run two different control algorithms, one based exclusively on the FF signal, and one based on the FB signal or both the FF and FB signals. One drawback of this digital approach is that it requires having high-speed digital electronics, such as very fast ADCs 11 with a sampling rate greater than 1 Msps (Mega sample per second), a very fast DAC 12, and a fast DSP 5, resulting in a high cost and a high power consumption by the OA control circuitry.
An object of the present invention is to overcome at least some of the shortcomings of the prior art by providing an apparatus for controlling the gain of an optical amplifier that is fast, stable and requires little electrical power to operate.